Shigella spp. is a Gram-negative bacterial pathogen that causes bacillary dysentery in humans by infecting epithelial cells of the colon. Shigella primarily infects intestinal epithelial cells (IECs). Shigella expresses several proteins that provide a mechanism for delivering effectors that induce bacterial uptake into the host cell via phagocytosis. To accomplish the injection of the effectors, Shigella use a type III secretion (TTS) system to induce their entry into epithelial cells and to trigger apoptosis in infected macrophages.
Bacteria of Shigella spp., including S. dysenteriae, S. flexneri, S. boydii and S. sonnei, are responsible for shigellosis in humans, a disease characterized by the destruction of the colonic epithelium that is responsible for 1 million deaths per year, mostly children in developing countries.
There are 15 serotypes in S. dysenteriae, 14 serotypes and subtypes are recognized in S. flexneri, S. boydii has 20 serotypes and a single serotype exist within S. sonnei although their prevalence is not evenly distributed. The most prevalent Shigella spp. in industrialized countries and of increasing prevalence in some Latin American countries is S. sonnei. S. dysenteriae type 1, which can produce Shiga-toxin, can cause high morbidity and mortality. S. flexneri is most prevalent in endemic region of developing countries. The World Health Organization (WHO) considered the development of a vaccine against shigellosis a priority for developing countries.
Although control and treatment of shigellosis outbreaks with antibiotics is possible, the high cost of antibiotics and the constant emergence of antibiotic resistant Shigella species, even to the newest antibiotics, underscores the need for an effective vaccine to help control Shigella and related enteroinvasive E. coli diseases in the developing regions of the world (12).
To establish a successful infection, Shigella finely regulates the host's immune response, especially those responses leading to inflammation. In contrast to Salmonella typhimurium, Shigella is inefficient at invading the apical pole of polarized intestinal epithelial cells. Instead, Shigella requires transmigration of polymorphonuclear leucocytes (PMN) to disrupt the epithelial barrier, facilitating cell invasion via the basolateral pole of epithelial cells (26). The host's inflammatory response, facilitated by cells of the innate immune system, attracts PMN to the site of inflammation. Therefore, triggering inflammation at the early stage of infection is required for cell invasion by Shigella. Bacteria that reach the intracellular compartment of the cells grow and spread from cell to cell, protected from host immune defenses. But, infected epithelial cells playa large role in the inflammatory process, both as sentinels that detect bacterial invasion and as a major source of mediators, particularly cytokines and chemokines that initiate and orchestrate mucosal inflammation. Recognition of the bacteria by the epithelial cells occurs essentially intracellularly via a cytoplasmic molecule, Nod1/CARD4 that senses a microbial motif, the peptidoglycan (8). Nod1 activation induces other proinflammatory signaling pathways including NF-κ3 and c-Jun N-terminal kinase (JNK) that lead to the expression of chemokines, such as interleukin 8 (IL-8). Thus triggering excessive inflammation is detrimental to Shigella's survival in the host.
Natural Shigella infections confer immunity and provide protection against subsequent infection with homologous virulent Shigella (5). This exclusively human disease is transmitted directly via the fecal-oral route from an infected patient or indirectly through contaminated food and water. It is a highly contagious infection, capable of transmission with as few as 100 microorganisms (6). Epidemiologic and volunteer studies have revealed that protective immunity against Shigella is directed against the LPS or O-specific antigen and is therefore related to serotype. Many approaches have been used for Shigella vaccines including use of live attenuated Shigella (16, 22), killed Shigella whole bacteria (18), and Shigella lipopolyssacharide (LPS) or O-polysaccharides conjugated to carriers such as proteosomes (24), tetanus toxoid (25) and ribosomes (31). Despite many years of extensive research, an effective and inexpensive vaccine against these Shigella species is not yet available.
The use of attenuated strains of Shigella as live oral vaccines has been demonstrated to induce protective efficacy. Results from the clinical trials of genetically well characterized, invasive Shigella vaccines are promising. CVD1208, SC602, WRSS1 add WRSd1 vaccine candidates, administered orally, are safe and immunogenic in volunteer trials and, in the case of SC602, have been demonstrated to protect against dysentery (11, 16, 17, 33). Clinical trials with CVD1208 demonstrated that the symptoms of mild fever and diarrhea, which are seen with some of the live Shigella vaccines, can be reduced by elimination of the sen and set genes from the vaccine strain. Duplication of a successful strategy in one serotype to other serotypes is an ongoing area of research but will eventually require use of a polyvalent mixture of Shigella strains of different serotypes that can protect against most of the Shigella (21). A recent multi centre study of Shigella diarrhoea in six Asian countries indicated that the relative distribution of Shigella species isolated from patients varied from different countries and sites. Moreover, S. flexneri serotypes were highly heterogeneous in their distribution from site to site, and even from year to year. The heterogeneous distribution of Shigella species and serotypes suggest that multivalent or cross-protective Shigella vaccines will be needed to prevent shigellosis worldwide (35). A vaccine that aims to confer broad-spectrum coverage would require inclusion of all of the important Shigella serotypes (21). To resolve this dilemma a vaccine strategy based on the use of ‘pentavalent formulations’, comprising S. flexneri 2a, 3a and 6 strains along with the attenuated S. sonnei and S. dysenteriae 1 strains has been advocated (Noriega et al., 1994). Alternatively, use of complex structures comprised of serotype-specific and cross-reactive antigens from Shigella, such as whole bacteria either killed or live-attenuated, could be considered as an approach to vaccinate against infections caused by the most common species and serotypes of Shigella. Intranasally administered Invaplex, a purified complex from Shigella water extract composed of the Ipa proteins and LPS, has been proposed and is in Phase 1 trials currently at the Walter Reed Army Institute of Research (WRAIR) (23).
There is thus a need for an effective vaccine to help control Shigella and related enteroinvasive E. coli diseases in the developing regions of the world.